H. f. electrical wave guide coupling arrangement



L. LEWlN Sept. '13, 1960 1-1.? ELECTRICAL WAVE GUIDE COUPLING ARRANGEMENT Filed May 27, 1957 5 Sheets-Sheet 1 L. LEWIN By Attorney H.F. ELECTRICAL WAVE GUIDE COUPLING ARRANGEMENT Sept. 13, 1960 3 SheetsSheet 2 Filed May 27, 1957 Inventor L. L WIN A t torn e y L. LEWIN Sept. 13, 1960 H.F. ELECTRICAL WAVE GUIDE COUPLING ARRANGEMENT Filed May 27, 1957 3 Sheets-Sheet 3 i l I N J tank Inventor L. LEW! N By A ttomey United States Patent H.F. ELECTRICAL WAVE GUIDE COUPLING ARRANGEMENT Leonard Lewin, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware This invention relates to high frequency electrical wave guide coupling arrangements particularly suitable for high frequency transmission systems in which a main energy guiding path is constituted by a tubular conductor of circular cross section along which energy is propagated in a circular H mode. When such a transmission system forms part of a long-haul or inter-city communication scheme the energy carried by the main guide may be distributed over a number of separate frequency bands, each band being allotted to a particular one of a plurality of communication channels: the frequency band of each channel being customarily utilised by a plurality of individually modulated sub-channels. The energy content of a channel is carried between the main wave guide and the associated branching channel terminating or processing equipment over a feeder guide which is coupled in to the main guide by what may be conveniently termed channel insertion equipment at a point which may be either at an end of the main guide,'or at an intermediate point therealong. In such a scheme the main guide may conveniently be op'erated :at millimetre 'wavelengths, at frequencies of the order of say' 30, 000 mc./s. to 60,000 mc./s., with channel spacings of the order of 1000 mc./s. or less, with a pass band per channel of width about 350 mc../s. I

The principal object of the present, invention is to provide a wave guide coupling arrangement'suitable for use, inter 'alia, in such a long haul wave guide transmission.

According to the present invention there is provided a high frequency electrical wave guide coupling arrangementcomprising a tubular metal main guide of circular cross-section, and at least one tubular metal feeder guide adapted for connection toa branch circuit and being "circu'mferentially disposed externallyto, said main guide so as to form a substantially complete turn therearound and having a wall effectively in common with said'main guide, said common wall having a plurality of substantially cophasally-acting coupling apertures cut therethrough, said apertures being spaced round a circumference of said main guide with at least one aperture per free space wavelength measured along said circumference at'the mean operating frequency of said feeder guide. "If directional operation is desired, an additional feeder guide may be provided, like unto that first mentioned, the two guides being spaced apart along the axis of the main guide by an odd number of quarter-wavelengths as measured in the main guide, the two feeder guides'being coupled to the same channel branch circuit through respective means having a relative transmission phase difference of 90, I w The invention will-be better understood from the following description of two embodiments, taken in conjunction with the accompanying drawings, in which:

, Fig. 1 is a diagrammatic represent-ation'of a longitudinal cross section of itmthe. invention; .I

Fig. 2 is a diagrammatic representation of a transverse cross-section of the arrangement of Fig. 1 taken at the line XX in the direction indicated by the arrows.

Fig. 3 is a diagrammatic illustration of a directional coupling arrangement according to the invention.

Fig. 4 is a perspective view of a directional coupler construction of the type shown diagrammatically in Fig. 3.

Fig. 5 is a curve referred to in explanation of the operation of arrangements according to the invention.

Fig. 6 shows a theoretical transmission line equivalent of a slotted feeder guide; and

Fig. 7 shows a theoretical network corresponding to Fig. 6.

Referring now to Figs. 1 and 2 conjointly, these illustrate diagrammatically the longitudinal and transverse sections of a waveguide coupling arrangement in accordance with the invention, all details other than those essential to an understanding of the invention being omitted. The arrangement comprises a length of main wave guide 1 in the form of a metal tube of circular cross section suitable for the propagation of high frequency energy in the circular H mode, as for example in a long haul or inter-city wave guide communication scheme. Exterior'to the main guide 1 is a hollow metal feeder guide 2, in this instance of rectangular cross-section, with the longitudinal axis of the feeder guide lying in a plane normal to the longitudinal axis of the main guide 1. The feeder guide 2 forms a substantially complete turn round main guide 1, and is so arranged that the two guides share a wall in common. In the present embodiment the shared wall of the feeder guide corresponds to a p n a n n WW M g V l P1; reference to Figs."1 and 2 depends on the fact that the the minor dimension b of the feeder guide rectangular cross section. Thisshared wall has a plurality of coupling apertures in the form of slots cut therethrough as indicated at 3, the slots being spaced round a circumference of the main guide with at least one slot per free space wavelength at the midband operating frequency of the feeder. In the illustrated embodiment the slots are spaced at intervals of one half of a feeder guide wavelength, and are arranged so that, as shown in Fig. 1, alternate slots have their respective longitudinal; axes inclined in opposite directions with respect to the minor dimension of the feeder guide. One end 4 of the feeder guide 2 is adapted for connection to a branch circuit, such as a channel input or output circuit, while at the other end the feeder guide is terminated by a short-circuiting device indicated at 5, which is preferably spacedfrom the slot nearest thereto by substantially one quarter of a feeder guide wavelength. Coaxially located within the main guide 1 is a cylindrical conductor 6, radially spaced from the wall of the main guide by a distance equal to between a half and a whole free space midband-wavelength Thev conductor 6 extends longitudinally with uniform diameter throughout that region of the main guide 1 in which the coupling between the main and feeder guide is being effected, but beyond that region the diameter of conductor 6 is gradually tapered at both ends to zero, as indicated at 7 for one end of the conductor. When a plurality of channels are coupled to themain guide 1 at respective proximate points therealong, the same conductor 6 may extend continuously through 'all the dilferent channel coupling regions in series without tapering to zero except at its ends. In such a case it may be desirable to taper or otherwise modify the diameter of the main part of conductor 6 just enough to maintain the optirmnn electrical spacing between the conductor-"6 and the wall of waveguide 1 at substantially the'same value in each of the coupling regions associated with the different. channels, no matter what their operating frequency. 7 1,;

The functioning of the. arrangement described :With

circular H series of wave modes differ from the other wave'modes which can be supported in a tubular metal guide of circular cross-section in that they exhibit no variation in field pattern round the guide, andthat they have their electric lines of force as circles in the guide cross-section. Thus, any peripheral feed round the .guide cross-section which excitesvcophasal azimuthal components of electric field at intervals more frequent than one per wavelength of circumference will excite only the circular H series of modes. In the described arrange- 'ment such cophasal excitation is obtained by energy transfer from the rectangular feeder guide, dimensioned to operate in the rectangular H mode, through the slots spaced at half-wavelength intervals and alternately inclined so as to maintain cophasality despite the change of phase of the energy as it is propagated along the feeder guide. By means of the coaxial cylindrical conductor 6 a restraint is placed on the generation of unwanted circular modes in the coupling region, and outside the coupling region the tapering of the cylindrical conductor results in the appearance of a substantially pure circular H mode in the remaining portions of the circular main guide 1. r

It will be recognised that the coupling arrangement described above with reference to Figs. 1 and 2 isof a non-directive nature i.e. energy introduced into the main guide by the feeder guide will be propagated in 'both directions along the main guide, while conversely the feed er guide will collect energy from the main guide irrespective of the direction of arrival of said energy. While such an arrangement may be useful in some cases, it is frequently desired that the coupling be of a directional nature, and this can be obtained in accordance with the present invention by means of an embodiment such as that illustrated diagrammatically in Fig. 3, and in perspective in Fig. 4. I

Referring to Figs. 3 and 4 conjointly, itwill be seen that this embodiment is similar to that illustrated in Figs. 1' and 2 but is provided with an additional slotted feeder guide 2' similar to feederguide-Z and slot coupled to the main guide 1 in the same manner as feeder guide 2, but spaced therefrom along the longitudinal axis of 1 present example the guides 2 and 2' are respectively cou' .pled to the colinear output side arms 8, 9 of a hybrid the main guide 1 by an odd number (in the present embodiment, three) of quarter wavelengths in the main guide 1. The two feeder guides are coupled to the same channel branch circuit in such manner that they operate with a relative phase difference of 90, this phase difference together with the spacing along the main guide 1 causing the feeder guides to cooperate for propagation along one direction of the main guide and to cancel each other for propagation in the reverse direction. In the waveguide junction network 10 such as that frequently referred to as a Magic T, through corner elementsll and 12. In order toobtain the 9 0 phase difference already mentioned, the hybrid junction 10 is displaced from V the centre. line between the twofeeder guides by one eighth of a feeder guide wavelength. The channel branch equipment (not shown) may be coupledito, the junction through either of the two perpendicular inputi arms 13,14. As indicated on the drawings, input arm 13 (per:

pendicular to the plane of the paper in Fig. 3) is parallel i .e. cophasally, coupled to the side arms .9 and 8, and if this arm 13 is used for feeding energy from the channel branching equipment then the excitation of feeder guide a 2 will be'retarded 90 relative to the excitation of feeder coupledinputxarm 14, there will be. a phase reversal in the arrow 16 in Fig. 3.

' branching equipments operated in the same frequency guide 2, and power will'flow along thejmain guide 1 only in the direction indicated by arrow,15 in Fig. 3. lf on the other hand theenergy is fed through the seriesband may be coupled each to'a respective one of the input arms 13 and 14, for receiving or inserting in both directions of the main guide simultaneously.

The frequency bandpass characteristics of coupling arrangements according to the invention depend on (a) the frequency variation ofthe coupling effect i. e. the variation of energy transfer between the feeder guide and the main guide. through the slots in the shared wall, and (b) the behaviour of the terminal impedance ofthe feeder guide with its array of slots.

'With' regard to the variation in energy transfer, it can be shown by mathematical analysis that if the number of slots N is large, say ten or over, the amplitude A of the circularHg wave induced in the main guide by constant amplitude excitation of the feeder varies with frequency according to a curve approximately of the form shown. at, 15 in Fig. 5, with A proportional to the expression I x=N.1r.dAg/2.Ag, the curve passing through a flat maximum at x=0, and through zero value at x=1r, 21r, etc. By centering the wanted channels at x=0, and undesired channels at x=1r, 21F, etc, mutual interference between channels can be avoided,

Since the spacing from the centre frequency of the wanted channel to the first zero corresponds to x=1r, we have since N=41rR/ Ag, Rbeing the radius of the main guide.

Now dAg/Ag=(df.Ag (f.A where f is the said centre frequency and A is the corresponding free space wavelength. 7 Hence,

, df/;f=A 21rRAg 2 As an example, if R the radiusof the main guide is 4 cms., and a the broad dimension of the rectangular feeder, then with f=37,500 mc./s., a=7. l mm., we have df=1000 mc./s. Thus a pass band of about 350 mc./s. is. amply catered for by the flat maximum at x =0 in .Fig. 5, while if other channels have their centre frequencies located, at 1000 mc./s. spacing, mutual interference is avoided. i p, r

:Turning now to the behaviour of the impedance of the feeder guide with its array of slots, in the first instance the slots themselves may be designedto be about a half wavelength long at the centre frequency. With a length of about 4 mm. and a minimum width of about 0.5 mm., their Q factor will be quite low, and unless the Q factor is raised by specialshaping ofthe slots (elg. dumb-bell) little selective effect can be expected except well away from the design frequency; However, theslot array impedance shows a frequency variability proportional to the number ofslots. a

Fig. ,6 shows an equivalent circuit of the array, consistingofN sections of length lfshunted by the slot impedance Z, with the last section left open circuited corresponding approximately) to the effect of the terminating short circuit referred to at 5 in Fig.2. Normalising all impedance values, and writing 21r/Ag=k', it

can beshown that the inputimpedance Z is given approximately by the equation V This equation holds in the neighbourhood Oran kn e from which it can be seen that the equivalent network is of the type illustrated in Fig. 7. This network comprises a load arm 17 of resistance Z/N shunted by a parallel inductance-capacitance arm 18 of reactance cot kl, the whole in series with a series-inductancecapacitance arm 19 of reactance j(N/3) (tan k'l).

Assuming from now on that Z has been chosen equal to N at the midband frequency, and remembering that, since the Q-factor of a tuned slot is fairly low, this equality can be assumed to hold over a range around the midband frequency, it will be seen that the equivalent network shown in'Fig. 7 consists of the end elements of aband-pass filter, which may be used in cooperation with other reactive elements inserted in the feeder guide prior to the slotted portion to complete a band pass filter, -adapted according to known technique to provide channel isolation.

In considering the use of such a band pass filter arrangement it should be noted that the loaded Q factor of the arms 17 and 18 as a function is given by 37,500 mc./s.

500 mc./s. :75

and Q =37.5

Solving 6 for R then gives R=l.2 cms. Such a radius is smaller than would be acceptable for the main guide as a whole, but if it is desired to use the filter technique as above described for channel separation, the small radius may be used for a coupling section of short length, with tapered sections connecting it to a guide of the radius suitable for the transmission characteristics desired for the main guide.

Still adhering to the filter technique, an alternative possibility to make the slot impedance Z equal to say N/ 3 instead of N. This has the effect of reducing the input impedance Z to the array by a factor of three, and also of reducing Q in Equation 5 by the same factor. This changes Equation 6 to Thus in order to retain the same value of Q the radius of the main guide must be increased by a factor of three, i.e. from 1.2 cms. to 3.6 cms., which may be a more practicable value. The reduction of the input impedance to the slotted guide may be overcome by inserting a suitable impedance-transforming wave guide section to feed the slotted guide at a point which is an even multiple of half guide-wavelengths from the first slot, any impedance discontinuity being tuned out in accordance with known technique.

Returning to Fig. 7, the series arm 19 of reactance i(N/3)(tan kl) may be shown to be of similar response to that of the shunt arm 18', but the magnitude of the impedance is insuflicient for a good band-pass filter. The reactance factor 1/3 needs to be raised to unity for a twoelement maximally flat filter, and to 2 for the centre element of a similar three-element filter. This may conveniently be done by inserting in the feeder guide, at a point about a quarter wavelength ahead of the slotted portion, as indicated by dots 19, 20 in Fig. 3 a tuned cavity electrically dimensioned to make up the deficit. Further cavities may then be added according to the design. Inthis way the frequency aspect of the channel insertion feed is built up. The disturbance presented to the main guide by such a feed outside its pass band will be sufficiently small to permit serial insertion or extraction of the various channels.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

' What is claimed is: l

l 1; A' high frequency electrical wave guide coupling arrangement comprising a tubular metal main guide of circular cross-section, and at least one tubular metal feeder guide circumferentially disposed externally to said main guide so as to form a substantially complete turn therearound and having a wall effectively in common with said main guide, said common wall having a plurality of substantially co-phasally acting coupling aper- Y tures cut therethrough and mutually spaced apart a distance equal to where Ag is the wavelength propagated in the feeder guide and means in said main guide for preventing the generation of unwanted circular modes, said means comprising a coaxial element, said element occupying a substantial part of the cross sectional area of said main wave guide in the region of said coupling apertures.

2. A high frequency electrical wave guide coupling arrangement comprising a tubular metal main guide of circular cross-section, a branch circuit, and at least one tubular metal feeder guide of rectangular cross-section, said feeder guide being terminated at one end and open at the other end for connection to the branch circuit and being circumferentially disposed with respect to the exterior surface of said main guide so as to form a substantially complete turn around said main guide with the longitudinal axis of said feeder guide in a plane perpendicular to the longitudinal axis of said main guide, said feeder guide and said main guide having a common wall, said common wall having a plurality of cophasally-acting coupling slots cut therethrough, said coupling slots being spaced around the circumference of said main guide at intervals substantially equal to where kg is the wave length in the feeder guide and means in said main guide for preventing the generation of unwanted circular modes, said means comprising a coaxial element, said element occupying a substantial part of the cross sectional area of said main wave guide in the region of said coupling apertures.

3. An arrangement according to claim 2, in which said common wall corresponds to the minor dimension of said rectangular cross-section, and in which said slots are spaced at intervals of one half said feeder guide wavelength, the longitudinal axis of any two adjacent slots being inclined in opposite directions with respect to said minor dimension.

4-. An arrangement according to claim 3, in which said feeder guide is terminated at one end by short-circuiting means spaced from'the nearest said slot by one quarter of a feeder guide wavelength, the other end of said feeder guide being provided with means for connection to said branch circuit.

5. An arrangement according to claim 4, in which the 7 m a for e tio rsaidh n h. c rc it ncl d sia b u Pa lt r h v n e e stp ca y. es ator l cated and 'dimensionedto provide channel filtering.

6. An arrangement according to c1aim'1,;"further con1 prising an additional feeder-guide like unto thatfirst mentioned and similarly disposed round said main guide,

'said additional feeder guide being longitudinally spaced selected according tothe desired direction of operation ofsaidmainguide. H v 8. An arrangement according to claim 7 in which two branch circuits are connected each to a respective one of saidinput arms for simultaneous operation over-said main guide indifferent respective directions. 4 4

l 9.. An arrangement according to claim 1, in which the means for preventing the generation of unwanted circular modes comprises a cylindrical conductor coaxially located within said main guide and extending along the coupling region of said main-guide, said conductor being radially spacedrfrom the wall of said main guide by a distance equal to between a half and one free space wavelength at said mean operating frequency. 7 j I a 7 10. An arrangement according to jclairn 9, w erein each end of said cylindrical conductor is tapered to zero diameter beyond the coupling region of said main guide.

- References Cited in the file of this patent V UNITED STATES PATENTS Raabe May 20, 1958 

